Lyme arthritis (LA), a common late manifestation of infection with Borrelia burgdorferi, usually resolves after appropriate antibiotic therapy. However, in some patients arthritis persist for months or years following apparent spirochetal killing by 2-3 months of oral and IV antibiotic therapy, termed antibiotic-refractory LA. Joints in these patients are characterized by excessive synovial inflammation and proliferation. MicroRNAs (miRNAs) are important regulators of various biological processes such as inflammation and proliferation, and mice with defects in miRNA function have significantly altered immune responses to B. burgdorferi infection and more severe Lyme arthritis. However, the role of miRNAs has not yet been examined in human LA. To determine how miRNAs modulate human LA, an unbiased screen was performed to measure miRNA expression in synovial fluid (SF) and synovial tissue from antibiotic-refractory LA patients. Patients with infectious LA (before IV antibiotic therapy), RA (a prototypic chronic joint inflammatory disease), or osteoarthritis (OA, a minimally inflammatory joint disease) were used as comparison groups. Synovial tissue and SF from patients with LA and RA, but not OA, contained an inflammatory miRNA signature, including miR- 146a and miR-155. In patients with antibiotic-refractory LA, this inflammatory miRNA signature correlated with elevated inflammatory markers and longer disease duration. In addition to an inflammatory signature, antibiotic-refractory LA and RA patients had a distinct proliferative miRNA signature, including miR-142 and miR-223. In contrast, infectious LA (prior to oral or IV antibiotic therapy) or OA patients lacked this proliferative signature. In vitro studies using fibroblasts, the predominant resident cell in the synovial lesion, demonstrated that miRNAs isolated from SF of LA patients are transferred to fibroblast-like synoviocytes, suggesting miRNAs may act as paracrine regulators of gene expression in resident cells during LA pathogenesis. Based on these preliminary results, it is proposed that after antibiotic therapy, patients with antibiotic- responsive LA appropriately down-regulate inflammation and repair damaged tissue, leading to resolution of arthritis. In contrast, patients with antibiotic-refractory LA fail to restore homeostasis following apparent spirochetal killing, and what starts as an immune response to infection develops into uncontrolled synovial inflammation and proliferation. In these patients, accumulation of inflammatory miRNAs indicates a failure to down-modulate immune responses, and accumulation of proliferative miRNAs indicates a failure to appropriately regulate proliferation and tissue repair. This proposal aims to determine the roles of miRNAs in regulating synovial inflammation and proliferation in human LA pathogenesis, and to explore the potential of miRNAs as novel biomarkers and therapeutic targets for patients with infection-induced chronic inflammation.